1. Field of the Invention
The invention is directed to a sensor for measuring electrical properties of a coating which is deposited on or eroded from a reference substrate to identify the resistance or the temperature of the reference substrate.
2. Description of the Prior Art
The limits of tolerance of specific properties such as, for example, layer thicknesses or sheet resistances of electrically conductive layers on semiconductor substrates are becoming narrower and narrower in the manufacture of thin film circuits and integrated semiconductor circuits. When, for example, such layers are composed of alloys that are manufactured by simultaneous vapor deposition or sputtering of two or more different targets, an exact knowledge of the composition of the layers and, consequently, an exact monitoring of the layer generating process become of utmost importance.
In chromium-nickel thin film resistors, the temperature coefficient of the electrical resistance must lie in the range of -25 to +25 times 10.sup.-6 /K Since this value is dependent on the composition, the composition must not deviate by more than plus or minus 3% atomic parts in the concentration of a constituent.
In the case of tantalum silicide layers which are utilized in very large scale integrated technology as a low resistance gate or as interconnect materials, the electrical resistivity i.e., the dependency of the resistance on the thickness of the layer, is also greatly influenced by the composition. Moreover, only layers with a very specific composition can form an SiO.sub.2 protective layer by oxidation of the silicon in an oxidizing atmosphere at temperatures of about 900.degree. C. Here, also, the oxide growth can only be reproduced when the concentration of a constituent in the tantalum silicide layer fluctuates only slightly about a fixed value, the deviation being a maximum of plus or minus 2 atomic percent.
An in situ identification of a relevant measured value, for example, the electrical resistance, during the production of the layers is desirable for exact supervision of the composition of a layer. Added thereto as a complicating factor is that the substrates to be coated must be moved during the coating in order to achieve uniform layers, for example, as by rotation or by planetary movement.
German Pat. No. 31 23 427, corresponding to U.S. Pat. No. 4,543,576, issued Sept. 24, 1985 and incorporated herein by reference, discloses an arrangement which is capable of measuring the electrical resistance and the temperature of a reference substrate introduced into an evacuated chamber of a coating system. A specimen having a defined geometry is applied to an independent measuring unit in the form of a double pallet. The resistance measured according to the 2-point or 4-point method is transferred to the outside as a digital signal by an electronic means in a pulse-code modulation by means of a transmitter. The electronic means, the transmitter, and an independent power supply, for example, a battery, are arranged in insulated fashion in the inside of the double pallet.
The described apparatus, however, has the disadvantage that the measuring unit has considerably larger dimensions than a wafer or substrate which, typically, has a height of 0.6 mm whereas the height of the measuring unit equals, for example, 15 mm. This consequently requires an enlargement of the load lock of the coating or etching system that is rigidly dimensioned in terms of its dimensions. Moreover, the measuring equipment does not have great resistance to temperature and can no longer be used for coating processes at temperatures above 90.degree. C.